A polymer, polyethylene terephthalate, PET, heavily used for the production of containers for liquids, is known. Its primary assets are transparency, low weight, release of forms allowing distinctive profiles based on commercial products or requirements, contrary to metal boxes, all of the same shape and same dimensions. It is the same for the containers that are produced from cardboard whose forms are limited. PET is unbreakable and has good mechanical properties of preservation and permeability, which makes it very attractive and explains for the most part its very heavy use.
These bottles made of PET are used for still liquids such as oils and mineral waters. In this case, the containers undergo only very few mechanical stresses. The PET is completely suitable. Actually, these liquids are cold filled without pressure.
These bottles are also used in the case of carbonated drinks and are therefore likely to pressurize the container. Tricks of design with grooves on the body of the bottle or so-called petaloid bottoms make it possible to enhance mechanical strength and/or resistance to pressure, without increasing the weight of the container in a detrimental way.
When the manufacturers need to hot fill a container, it then is necessary to use different designs that require larger thicknesses, different geometries including panels placed on the body of the container to produce beams. These elements that are necessary for hot filling lead to high weights with high material consumptions, up to two times the weight of the same bottle for cold-filled liquids.
Actually, the mechanical characteristics of the PET deteriorate greatly when the temperature rises. There are so-called “heat-resistant” processes, more commonly designated by the letters HR, which make it possible to improve the heat resistance of the container that is thereby produced. A first so-called one-wheel process makes it possible to reach filling temperatures of 80/88° C. A second so-called two-wheel process makes it possible to package the liquids at temperatures of 88/95° C.
A hot-filled bottle actually undergoes numerous mechanical stresses during different phases. Thus, the bottom is to withstand the hydrostatic pressure of the hot liquid during filling. The container is to withstand forces produced by the evacuation caused by the cooling of the liquid when the container has been plugged when hot to ensure the sterile nature of the liquid. The cooling causes a double contraction, that of the liquid and that of the air of the top space of said bottle. It is for this reason that the profiles are much more complex with panels and beams on the body, bands marked on the body as well as a shoulder between the spout and the body, whose shape is rather bulbous. The advantage of the thickness that is necessary to the mechanical strength is also having a higher inertia at the temperature.
The manufacture of light bottles made of PET uses the so-called extrusion/blow molding process. This process consists in making a preform by extrusion, whereby this preform has a tube profile with one end formed to dimension and to the definitive form of the spout, and whereby the other end is closed. After heating this preform, in particular by infrared radiation, up to 100/120° C., the amorphous material is softened and can undergo blowing through the interior after it has been placed in a suitable mold. This mold has dimensions such that the withdrawal of the material with cooling is taken into account so that the final container has the desired dimensions.
During this blow-molding phase, a longitudinal stretching occurs under the action of a stretching rod and inflation by the pressurized air that is thus introduced. More precisely, the air is first introduced at low pressure to ensure a suitable deformation of the material during high amplitudes then at high pressure to ensure plating against the walls of the mold during finishing and for very low amplitudes. The molds are also cooled with water so as to dissipate the calories transmitted by contact, which also has the effect of immobilizing the bottle.
Actually, the bottles that are thus obtained are called bi-oriented because they have undergone stretching in one direction and an omni-directional inflation. The macromolecular chains that are thus oriented in two directions lead to excellent parameters of mechanical strength, at ambient temperature. The drawback of this bi-orientation is being in part reversible, and the material thus regains a certain freedom as soon as the temperature rises. Actually, the material has a tendency to return to its initial form in which it has fewer stresses. It is the so-called shape memory phenomenon.
For the thick bottles that are designed to be used for hot-filled drinks, use is also made of extrusion/blow molding, but with more sophisticated and more complex behavior parameters. Actually, the preform is heated to a higher temperature than in the case of light containers, close to the crystallization so as to reduce this PET shape memory and to relieve the stresses due to the blow molding.
In the case of manufacture with one wheel, so as to increase its strength at temperature, the initially amorphous material of this container is made to undergo a heat treatment, during and after its shaping. The material, when it is stretched after softening, generates an induced, but reversible, crystallinity, whereby the material remains transparent. The mechanical properties are enhanced. Thus, if the heating is maintained after having generated this induced crystallization, a spherulitic crystallization occurs, causing a certain crystallinity of chains that are already organized by bi-orientation. Contrary to the direct spherulitic crystallization of the PET, the spherulitic crystallization subsequent to a bi-orientation perfectly preserves the transparency of the material.
In the case of two-wheel manufacture, the process makes it possible to reach higher performance levels, but at the cost of a succession of more complex stages. Actually, in this case, a blank of much larger volume than the volume of the final container, two to three times as much, i.e., with a proportional stretching rate, is first worked up. This blank is then heated beyond the vitreous transition to relieve the stresses, which brings about a reduction of volume and a return to the dimensions of the preform, but with a high rate of spherulitic crystallinity, whereby this leads in a proportional way to a homothetic container. There is self-regulation with the PET.
When this restricted blank is at temperature, a blowing stage with a mold with the dimensions of the final container to be obtained, aside from recesses, makes it possible to manufacture the final container. The high rate of crystallinity imparts to this container an improved resistance to hot filling. It is noted that such a process is much more burdensome to put into place. The process requires behavior always at the limits of values, requires cleaning of molds, as well as intensive and regular maintenance.
In addition, it should be noted that the bottles that are obtained by the HR process have a tendency to absorb water as soon as they are manufactured, which reduces their characteristics of mechanical strength and therefore temperature resistance. It thus is possible to obtain manufacture of a container that initially withstands a temperature of 88° C. and that, after uptake of water, withstands only 82° C. Actually, the transition temperature TG drops.
Whereby storage should be reduced as much as possible, the bottles are generally produced on the filling site, for just-in-time use, which is another constraint. Once these containers are manufactured, there are several filling methods and various properties of the liquids to be packaged. There are liquids that are sensitive to light, such as milk or beer, sensitive to oxygen absorption and therefore oxido-sensitive, such as fruit or vegetable juices, beer, oil, but also sensitive to the uptake of water, to the loss of gas, to the development of yeast, mold or bacteria. The liquids can include preservatives and are thereby not very sensitive; in contrast, certain so-called still and delicate liquids—such as milks, juices, coffee, tea, fruit drinks., and certain waters—do not include any preservative and should still be packaged under the best conditions.
To ensure such packaging under conditions of suitable hygiene and with all of the guarantees of good preservation, two primary methods are known: one called “aseptic filling,” and the other called “hot filling.” The aseptic filling is simple in theory because it consists in filling the container with a sterilized liquid and in plugging said container, whereby the packages just like the plugs are sterilized, and the operation is conducted in a sterile environment in its entirety. Nevertheless, it is understood that the chain is complex to install, difficult to keep always under the same aseptic conditions over time, require a very high monitoring and high maintenance producing high costs. In such a chain, it is necessary to use chemical sterilizations that use chemical products with treatments that are derived therefrom, expertise of personnel, and low yield due to treatment speeds that are not very high. The yield is 40 to 50% of that of a hot filling chain. The investments are also very large, two to three times larger than that of a hot filling chain.
A very significant drawback of this process resides in the impossibility of monitoring online the sterility of the contents in each container. At the very most, the monitoring can be done by sampling. The advantage of this cold aseptic filling is to require only thin-walled bottles of low weight and of free form since the cold filling prevents the deformations due to the temperature.
The other method, hot filling, also guarantees a quality of asepsis, since the monitoring of the temperature of the contents is simple and easy at any time. The bottling line is simple, and the treatments of the container and the plug are limited in scope since the sterilization is obtained by the hot liquid itself, introduced into the container that is immediately closed after filling. A tipping of the bottle also ensures the sterilization of the inside surface of the plug in contact with the liquid. In contrast, it is necessary to use containers that are able to withstand the filling temperature of between 60 and 95° C., more particularly between 80 and 92° C., based on the products. In addition, the bottles have high weights with approximately identical shapes linked to the resistance constraints, which allows only a very slight differentiation between the marketed products.
Also, it is concluded that there are two processes that have advantages and disadvantages. Nevertheless, the additional expense produced by the particular characteristics of the containers currently used and necessary for hot filling tend to orient the manufacturers involved toward the activation of filling lines by the aseptic method.
It is important to set an estimate of the material weight. Fifteen years previously, a 1.5 liter container required 49 g of cold filling material and 55 g of hot filling material, HR treatment. Since then, important gains have been made for the cold filling reaching 28 g, while the amount of material for hot filling has stayed almost the same.
The compromise sought by the manufacturers would consist in being able to fill hot liquids to obtain the guarantee of asepsis but in thin-walled bottles that are designed for cold filling to limit the costs of the containers as well as the packaging line. This is what the process according to this invention proposes, which is now described in detail according to a preferred, nonlimiting embodiment.